U.S. Pat. No. 6,756,139 described a coated conductor architecture based on ion beam assisted deposited magnesium oxide (IBAD-MgO), which comprises four or five separately deposited layers between the metal substrate and the superconducting layer. Among the described structures are included: (1) a structure including a substrate/a layer of aluminum oxide (Al2O3)/a layer of yttrium oxide (Y2O3)/a layer of IBAD-MgO/a homoepitaxial layer of MgO/a layer of strontium ruthenate (SrRuO3)/a layer of yttrium barium copper oxide (YBCO); and (2) a structure including a substrate/a layer of erbium oxide (Er2O3)/a layer of IBAD-MgO/a homoepitaxial layer of MgO/a layer of SrRuO3/a layer of YBCO. Other architectures have employed strontium titanate in place of strontium ruthenate or mixtures of strontium titanate and strontium ruthenate.
Excellent critical current density values have been achieved using either of these architectures, but layers of strontium ruthenate and/or strontium titanate may present unwanted difficulties in practical commercial processes. Specifically, while benchscale research on such materials generally employed pulsed laser deposition (PLD) that produced high quality stoichiometric films, PLD has not been preferred for commercial production of coated conductors due to high capital costs of laser equipment and relative small deposition areas. Other deposition processes such as sputtering are generally desired for commercial production, but multicomponent films of strontium titanate or strontium ruthenate are not optimally deposited by sputtering.
Due to these problems, a substitute material for multicomponent films such as strontium titanate or strontium ruthenate is desired. Among the requirements for such a replacement material are the option of using sputter deposition of a metal target as opposed to a metal oxide target and the preference for a single component metal target rather than an alloy of metals such as strontium and titanium.
After careful experimentation by the present inventors, a substitute material of hafnium oxide has now been found for strontium ruthenate or strontium titanate.